AquaFlux™ Model AF200

(a) What is TEWL ?

(b) How do you measure TEWL ?

Illustrated on the left are three instruments (evaporimeters) that are commonly used for measuring TEWL. They use three different measuring methods as described below.

(i) Open-chamber Method

(ii) Condenser-chamber Method

(iii) Unventilated-chamber Method

References

(a) How do they correlate ?

(b) How do the readings compare ?

Although the measurements correlate well, the readings generally differ. This is due to calibration differences, with no agreement among manufacturers about best practice. The calibration disagreement in the above study was found to be quite small, ~14%, but disagreements by a factor 2 or more are not uncommon. Such disagreements can be resolved by the traceable droplet calibration method developed in collaboration with the UK National Physical Laboratory.

(c) Where’s the difference ?

How this translates into practice is illustrated in the figure below, where two skin sites of the same volunteer were repeatedly tested in alternation.

References

A comparison between an AquaFlux™ Model AF102 and a Delfin VapoMeter was published as a poster at the ISBS Meeting in Philadelphia in 2005 [1]. Here we show a similar comparison with updates to illustrate the performance of our latest  AquaFlux™ Model AF200.

Below are the results of an experiment consisting of 200 repeat measurements using an upside-down wet-cup source with each instrument. More…

Below is a table summarising the scatter of readings observed in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. More…

The AquaFlux™ records ~2 flux readings per second. TEWL measurement speed depends on skin condition and software settings. Flux readings settle quicker with dry, well acclimatised skin than with moist skin. Software settings determine how precisely the flux readings need to settle before a TEWL measurement terminates. The default criterion is a standard deviation of 0.075 g/(sq.m h) in a running average over the last 10 flux readings. You can adjust these values to trade off precision for speed. There is no waiting time between measurements – you can site-hop.

The VapoMeter measures vapour accumulation rate rather than flux. A typical contact time of ~10 seconds is required to produce a reading. This is followed by a recovery period of up to 90 seconds, where the measurement chamber needs to be voided of accumulated water vapour before the next measurement can begin.

AquaFlux™ and VapoMeter measurement times were compared in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. Average repeat-times worked out to ~47 seconds for an AquaFlux™ Model AF200 and 38 seconds for a VapoMeter.

(d) Quality control

You’ve finished a study and are pondering the results. You spot something unexpected and are not sure how to interpret it. With the AquaFlux™ you can inspect the recorded flux curves and other supporting data. With the VapoMeter you’re stuck. Which one gives you more insight into what went on? Which one may save you having to do a repeat-study?

(e) Versatility

References

The AquaFlux™ determines TEWL from water vapour flux density measurements performed at a rate of ~2 per second. When you first place the measurement head onto the skin, the conditions in the measurement chamber are perturbed and it takes some time for the instrumental readings to settle. From then on, the software is in control, looking for the end-point that meets your accuracy requirements.

Normal TEWL Measurement

Normal TEWL measurements on acclimatised, healthy skin are quick, because there is no skin surface water (SSWL) to slow things down. According to the TEWL guidelines for open-chamber instruments [1, 2], you need to wait for the probe to recover after every TEWL measurement before starting the next measurement. This is not necessary with the AquaFlux™. You can go from site to site without pause and get the job done with maximum efficiency.This in-vivo site-hopping technique was first tested with a now obsolete Model AF102 AquaFlux™ in a series of measurements on 7 sites of a volar forearm [3]. The updated measurement times using a Model AF200 AquaFlux™ were found to be as follows:-

Accuracy & Measurement Time

As indicated above, measurement time can be traded for accuracy. If speed is important, then you can set less stringent TEWL endpoint settings in the software. The effect of this is illustrated in the three screenshots below.

StDev = 0.075 g/(sq.m h) over the 10 most recent flux density readings.

How this looks in practice is indicated in the uppermost screenshot. These settings were used for determining the measurement times in the table above.

If you want quicker measurements, then you can use a larger StDev setting. This is illustrated in the two lower screenshots with StDev settings of 0.3 and 0.5 g/(sq.m h). Clearly, the flux readings have not settled, but if you use the same settings throughout a study, then the relative TEWL values will be meaningful. These StDev values are typical for open-chamber instruments. The higher sensitivity of the AquaFlux™ makes them look inappropriate.

References

(a) Probe angle

According to published TEWL Guidelines [1, 2] open-chamber instruments such as the C+K Tewameter can only be used reliably on horizontal surfaces.

Unventilated-chamber instruments such as the Delfin VapoMeter have also been found to be affected by probe angle [3], although the manufacturer claims orientation-independence.

For the condenser-chamber AquaFlux™, detailed measurements show that the probe has an orientation-dependence that is non-symmetrical, depending on whether the RH and T sensors in the wall of the measurement chamber are above or below the chamber axis [4]. For negative angles, where these sensors are below the chamber axis, probe sensitivity was found to decrease by as much as ~6% relative to the calibrated value. For positive angles, where these sensors are above the chamber axis, probe sensitivity was found to remain within about ±1% of the calibrated value. The probe can therefore be used with all surface orientations with little effect on sensitivity, as long as its inclination is confined to the positive semi-circle.

(b) Contact Pressure

In practice, relatively large contact pressure effects have been reported for open-chamber instruments [6, 7]. Guidelines [1, 2] recommend that the contact pressure of the probe onto the skin should be kept low and constant, with measurements within a series preferably performed by the same operator.

For the unventilated-chamber VapoMeter, De Paepe et al [5] observed no statistically significant change of TEWL readings (from 7±2 to 8±3[g/(m²h)]) when the contact pressure was increased.

The effect of contact pressure on condenser-chamber AquaFlux™ TEWL measurements was assessed in an experiment using a spring balance to measure contact force. Measurements of mid-volar forearm TEWL were repeated 18 times on the same site with contact forces progressively increased then reduced in the range 0.1 to 2kg. The average TEWL over all 18 measurements worked out to 8.23[g/(m²h)] with a CV of 1.7% [4]. It appears, therefore, that the effect of contact pressure on TEWL measurement has more to do with measurement head design than skin barrier property change.

(c) Atmospheric Pressure

The effect on open-chamber TEWL measurements was first discussed by Nilsson [6]. He concluded that, at a given location, weather-related changes of atmospheric pressure could affect TEWL readings by as much as ±6%. This was deemed to be too small for further consideration.

The effect on condenser-chamber AquaFlux™ and unventilated-chamber VapoMeter instruments was investigated by Kramer er al [8]. For the AquaFlux™, measurements showed a clear trend of increasing TEWL readings with increasing atmospheric pressure (gradient = [150±22]x10^-4 [g/(m²h)]/hPa). No trend with pressure was apparent in the VapoMeter measurements (gradient = [13±44]x10^-4 [g/(m²h)]/hPa), although a weak dependence cannot be ruled out, given the relatively large standard deviation of the gradient.

(e) Probe Temperature

Unventilated-chamber VapoMeter readings have also been found to be strongly temperature dependent. By holding the VapoMeter between both hands, De Paepe et al [5] caused its temperature to increase by ~6°C and observed an increase of volar forearm TEWL readings from a baseline of 7±2[g/(m²h)] to 15±6[g/(m²h)], which works out to a temperature coefficient of ~1.3±1.1[g/(m²h)] per °C temperature change.

By contrast, the condenser-chamber AquaFlux™ probe shows little effect upon heating. A typical figure, calculated as the root-mean-square (RMS) average over 10 instruments, is ~0.05±0.06[g/(m²h)] per °C temperature change [4].

(g) Skin cooling

(f) Skin drying

References

There are two published guidelines for TEWL measurement in general [1, 2], and one for TEWL measurement in non-clinical settings [3]. Guidelines [1, 2] consider open-chamber instruments only. Guidelines [3] consider both open-chamber and unventillated-chamber instruments. But none of these publications included any AquaFlux™ measurements.

Guidelines [1, 2] discuss (a) person-linked variables, (b) environmental variables and (c) instrumental variables in some depth before presenting their recommendations for best practice. Most of the considerations of sections (a) and (b) are valid for all methods of measurement. The exceptions are section (a) of [2], where the discussion of skin surface temperature includes consideration of how this may affect the measurement probe, and section (b) of both both publications, which include discussions of air circulation and how this may cause fluctuations in open-chamber measurements. Neither of these are concerns for the AquaFlux™. Section (c) and elsewhere is instrument-specific and not applicable to the AquaFlux™.

The table below presents a side-by side comparison of the recommendations of these guidelines with what we recommend for the AquaFlux™.

Our answer is that TEWL does indeed give a useful measure of barrier function because a good skin barrier will let through less water than a poor skin barrier.

The difficulty arises because TEWL instruments do not measure TEWL directly. In fact, TEWL instruments are evaporimeters and they measure the flux of water vapour coming from the skin surface. You can interpret this measured water vapour flux as TEWL, if two conditions are satisfied, as follows:

TEWL and nothing but TEWL.

TEWL may not be the only source of water vapour flux. Other possible contributions include perspiration, evaporation of free surface water and leaks in the apparatus. The measured vapour flux can be related to the skin barrier only when these non-TEWL sources have been eliminated.

All must evaporate.

If you’ve satisfied that (1) above is OK, then TEWL will be your only source of water vapour. But remember that TEWL is the migration of condensed water through the epidermis, whereas TEWL instruments measure vapour flux from the surface. All the water arriving at the skin surface must evaporate for the vapour flux to be equal to TEWL. If the humidity in the measurement chamber is too high, then this won’t happen.

The AquaFlux™ is ideal for TEWL measurement, because it maintains a low microclimate humidity at the skin surface, irrespective of ambient conditions. This satisfies condition (2) above better than any other instrument on the market. It also helps with condition (1) by (a) providing the low humidity conditions necessary to get rid of any free surface water quickly, and (b) providing reliable, leak-free means to couple the measurement head to the surface of interest, including Franz cells [2].

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